Re: freshmen physics problems



On Nov 4, 4:30 am, "will" <u...@xxxxxxxxx> wrote:
Hi, I have two poblems from freshman Physics that I am puzzling over.
If anyone has insight into these I would be grateful to hear from you.
First, in a problem concerning circlular motion, a car moving in a
circular non-banked path. It was said in Sears and Zemansky that since
there is no motion in the radial direction (because the car in moving
uniformly in a circle) the frictional force that is required to keep
the car in a circular path is static and not kinetic friction. That
seems like an odd way to see it. I understand that the force is being
applied in the radial direction and since their is no change in radius
that we can say that the (frictional) force must be acting in the
radial direction where there is no movement but still it seems strange.
I guess the best way to resolve the issue would be if it were possible
to measure the frictional force and see if it is closer to that gotten
when a static coefficient is used as compared to that which is gotten
when a kinetic coefficient is used. Has this been done? Is there a
clearer way to see this?

Yes, there is a simple way to see this.
Kinetic friction is when two surfaces slide over each other -- a
slipping action. When a tire rolls, it doesn't slip on the road. The
bottom surface of the tire meets the road but does not slip on it.
When a tire *skids* (as when you stomp on the brakes and lock the
wheels), that's kinetic friction. And since kinetic friction is
typically lower than what you can accomplish with static friction, this
is good motivation for anti-lock brakes.


The second question is with regard to apparent weight in a moving
elevator. A 50 kg woman stands on a bathroom scale while riding in the
elevator. The elevator is originally moving downward at 10 m/s. It
slows to a stop with constant acceleration in a distance of 25 m. What
is the reading on the scale during the acceleration. We can do this
problem by looking at the normal force exerted by the scale on the
woman since it is equal in magnitude to the force the woman exerts on
the scale by the third law. In that case the second law provides us
with the fact that the sum of the vertical forces exerted on the woman
are the normal force plus -mg and that this sum is equal to ma. We can
then solve for n since the other values are known.
But it is not necessarily intuitive to focus on the forces on the
woman. It seems more intuitive to focus on the scale and the force(s)
acting on it. In that case we have the forces acting on the scale are
the woman's weight , whose magnitude is again -mg. But at this point I
am confused about what to do from here.

The question is what the scale *reads*. What the scale reads is the
force it has to apply to the object that's sitting on it. If, for
example, the scale operates according to Hooke's law, then there is
some deflection or compression that is proportional to the force the
scale exerts, and it is this deflection or compression that the scale
reports (though the numbers on the scale are calibrated to force units,
not deflection units).

When you gently stand on a scale, it tells you what force it has to
provide to keep you from penetrating it. In this case, the force it
provides happens to equal your weight, since by doing so, it puts you
in equilibrium.

When you jump on a scale from a small stool, however, the scale has to
provide more force because it also has to slow you down. This is why
the needle jumps so sharply. You don't weigh anything more, but in this
scale the force the scale provides is not equal to your weight.


Thanks for any help.

.

Relevant Pages

  • Re: freshmen physics problems
    ... there is no motion in the radial direction (because the car in moving ... to measure the frictional force and see if it is closer to that gotten ... A 50 kg woman stands on a bathroom scale while riding in the ...
    (sci.physics)
  • Re: freshmen physics problems
    ... there is no motion in the radial direction (because the car in moving ... and the stsic rather than dynamic coefficient of friction is applicable. ... A 50 kg woman stands on a bathroom scale while riding in the ...
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  • Re: freshmen physics problems
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